1. Technical Field
The invention relates to a base material for an aluminum offset printing form (plate) consisting of from about 0.2 to about 0.6% by weight iron, less than about 1% by weight manganese, and less than about 0.25% by weight silicon and copper, the rest being aluminum and impurities occasioned by production, and also a process for producing such base material.
2. Background and Prior Art
There is a need for offset printing plates which have an increased usage life (print run). The increased usage life can be achieved by burning in or hardening the photographic image on the plate for a few minutes at 230.degree.-240.degree. C. As a result of this treatment, the plates become very resistant to abrasion.
Pure aluminum is predominantly used in the manufacture of printing plates as, for example, described in Aluminum-Taschenbuch, 14th edition, page 109. These plates can be electrochemically roughened or grained in HCL baths and also in HNO.sub.3 baths to produce a bright, uniform surface appearance. However pure aluminum suffers a considerable loss in strength as a consequence of the heat generated during the burning in so that this material cannot fulfil the increased demands on the print run.
Some plate manufacturers are therefore using an AlMn alloy AA 3003 which contains approximately 1% Mn and is thermally much more stable than pure aluminum. Offset printing plates composed of manganese-containing alloys are also known from EP-A-0,164,856. This patent discloses aluminum alloys containing from 0.05 to 1% by weight of Mn, 0.02 to 0.2% by weight of Si, and 0.05% to 0.5% by weight of Fe. These manganese-containing alloys have the disadvantage that while the surface can be satisfactorily roughened in an HCl bath with modified process parameters, the surface is very poorly (nonuniformly) roughened in an HNO.sub.3 bath. In both acid systems, a dark coating is produced on the surface which has an adverse effect on the printing properties. In EP-A-0,164,856, no information is given concerning the roughening behavior of the AlMn material in an HCl bath on the one hand and in an HNO.sub.3 bath on the other hand, or on the roughening behavior as a function of the manganese content.
Processes for producing an aluminum base sheet for printing plates by treating the surface with an aqueous chloride-containing electrolyte solution followed by subsequent anodization is known in the art, as for example disclosed in German Patent Specification 2,557,222.
Accordingly, an object of the present invention is to provide a base material for aluminum offset printing plates and a process for producing same which, while having good thermal stability under burning-in conditions after roughening in HCl or HNO.sub.3 baths, has a more uniform and brighter surface than conventional manganese-containing aluminum alloys and which is comparable with the plate surface quality which can be achieved with pure aluminum.